Physics Photo of the Week

September 30, 2011

Pileus cloud
Notice the small wispy cloud at the top of this large cumulus cloud.  The cumulus cloud is developing into a small thunderstorm.  Such clouds are  formed from strong thermal convection.  When the vapor-laden air condenses to form the cloud (due to adiabatic expansion as the air rises), the large latent heat is released giving the cloud more buoyancy and driving the cloud tops even higher.  Because these clouds contain rapidly rising air masses, some of the drier high altitude air that is not part of the cumulus cloud gets pushed to higher altitudes ahead of the cloud.  The resulting condensation of the drier air on top of the cloud forms a wispy cloud of ice crystals on top of the cumulus cloud.  This small wispy cloud is called pileus after the latin word for "skull cap".

The animation sequence shown at right shows the vigorous upward movement of the cloud.  The pileus "cap" shows only briefly in a couple of frames of the animation sequence.

Thunder was heard in this storm cloud during the photo session, and a flat "anvil" top was also seen - but soon obliterated by another storm approaching from behind the camera.  No rain fell on my location from either storm.

This animation was taken by setting the interval timer on the camera to take a picture once every 10 seconds.  The animation is played back at 20 frames/second - a speed up factor of 200.  The photographs were taken about one year ago (September 22, 2010).

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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